The present invention relates to a filter for operation with acoustic waves.
In electrical technology it is known to use filters which operate with elastic, acoustic waves. These consist, in particular, of surface wave filters. Filters of this kind are described in general terms in Mathews "Surface Wave Filters", J. Wiley & Sons, New York, incorporated herein by reference.
A filter of this kind is constructed on a preferably piezo-electric substrate. It has at least one input transducer, at least one output transducer, and possibly additional structures, such as in particular reflector structures. Reflector structures of this kind are used in particular for resonator filters, in which, in particular, the input transducer and the output transducer can also represent one and the same transducer. In order to achieve a specific, predetermined transformation characteristic and a specific, predetermined resonance characteristic, it is known to weight the transducers, which are designed as interdigital structures, i.e. to provide the transducers with a finger weighting (finger-length weighting, finger position weighting, or omission weighting). A transducer such as that used in the present invention normally consists of an interdigital structure composed of two interlocking combs. Such a comb consists of fingers which are arranged parallel to one another and which, in the case of a transducer, are electrically connected by means of a busbar. As is known, finger-length weighting comprises a greater or lesser overlap between adjacent fingers. One of these fingers is assigned to the first comb and the other finger is assigned to the other comb. Arrangements comprising split fingers are similar. The length dimension of the mutual overlap between two adjacent fingers of this kind is a gauge of the electro-acoustic efficiency of this pair of fingers. Adjacent fingers, of which one is assigned to the first comb and the other to the other comb, are spaced from one another by intervals on the order of half a wavelength of the elastic, acoustic wave which is to be produced.
FIG. 1 shows a filter 1 with three transducers arranged on a substrate 2. For example, the transducer 3 arranged in the center represents the input transducer, and the two outer transducers 4 and 5 represent output transducers, possibly connected in parallel. In the substrate 2, elastic waves produced by the input transducer 3 pass in the main propagation directions 6 and 7 of these waves to the output transducers 4 and 5.
The acoustic waves in question include surface waves in a narrow sense (Rayleigh-Waves; Bleustein-Waves), transverse and shearing waves running close to the surface (such as surface skimming bulk waves), and also components running close to the surface of (longitudinally polarized) volume waves. In the following description, these types of waves will be referred to as surface waves (to be understood in a more general sense). The type of wave which will be produced in a particular situation will depend upon the techniques, known to those skilled in the art, relating to the dimensioning of the transducers, and possibly also upon anisotropic properties of the substrate.
FIG. 1 illustrates the transducers, in particular the input transducer, from which a generated acoustic wave is transmitted, and having side edges 8, 9 which are parallel to one another. These side edges are busbars aligned parallel to one another, and a correspondingly constant width of the transducer surface. Across the entire transducer, this constant width results in a constant length dimensioning for the arrangement of the fingers. Such a design of a transducer has the advantage that the waves emanating therefrom do not collide with busbars extending into the path of propagation, which is a disadvantage of transducers comprising convergent busbars.
A wave which is generated in the input transducer 3 and transmitted from the transducer 3 in accordance with the main propagation direction 6 and 7 is subject to disturbances even when the design of the interdigital structure of the transducer is precise. These disturbances are based on a discontinuity effect governed by the fact that the wave which is produced in the transducer and is transmitted therefrom leaves the transducer at its end and passes into a free portion of the substrate. An abrupt transition occurs between the structure of the transducer and the free surface of the substrate.
For many years it has been known (e.g. from the Japanese Patent application No. 56-122215A, incorporated herein by reference), to eliminate such disturbances by employing a technique whereby the transducer is equipped at the end in question with fingers which become shorter in stepped fashion so that the end in question of the transducer has a stepped termination. The efficiency of this technique is fundamentally dependent upon the fineness of the steps, but in practice cannot result in a complete elimination of the discontinuity effect. In particular this technique has the disadvantage that it operates only in an extreme frequency selective manner.